Torque limiting devices in brake actuator clutch mechanisms are well-known. The known devices can be divided into four basic categories. The first category includes devices utilizing a tightly wrapped coil spring to act as a torque limiting device. This design uses a precision machined, or formed, spring that acts on the internal diameter of a clutch sleeve. Devices in this category are disadvantageous since they typically require both a rotational movement and a linear movement to install and they are comparatively bulky in radial cross section.
The second category of devices are those that utilize a clutch pack assembly as the torque limiting device. This design uses a series of clutch plates, a splined inner and outer clutch sleeve, a compression spring to develop the clamping force on the clutch pack, and a variety of retaining and assembly parts. This design is disadvantageous since it typically requires a high number of components and it requires a large amount of space.
The third category of devices uses a ball spring torque limiting device. The ball spring torque limiting device is a highly machined and precisely designed structure with complex components, complex assembly and complex operation.
The last general category of devices uses a slip sleeve to wear surface torque limiting device. A compression spring produces a force that acts on a hardened sleeve that is in contact with a wear surface located on the rear of the tappet sleeve. The compression spring is retained by a custom-made quarter twist internal hex locking nut. The nut locks in a machined slot of hex cross section in the tappet sleeve. Devices in this category are known to have limited reliability.
Various embodiments of the devices in the above-described categories have been patented. For example, U.S. Pat. No. 4,351,419 teaches an adjustable mechanism having a first coil clutch and a second coil clutch. The clutches act together to permit adjustment of the mechanism to a predetermined torque, but slip if the predetermined torque is exceeded. More specifically, the clutches permit selective rotation of a nut member and an intermediate sleeve.
U.S. Pat. No. 4,394,890 teaches an adjustable clutch mechanism similar to the '419 patent in that two clutches are used. One of the clutches is a force limiting coil clutch that transmits clockwise torque from the intermediate sleeve to the driven sleeve. When the friction linings are not in contact with the disc, the torque transmitting capacity of the clutch is greater than that required to rotate the nut member. But, when the friction linings are in contact with the disc, the torque transmitting capacity is less than the torque required to rotate the nut member. The coil clutch thus acts as a torque limiting coupling for clearance sensing purposes in the low torque transmitting direction of rotation.
U.S. Pat. No. 4,527,683 provides for a torque limiting helical coil clutch drivingly connecting a first rotatable member and a second rotatable member. The coil clutch is right-hand wound to radially expand when transmitting a clockwise rotation and to radially contract when transmitting a counterclockwise rotation. The first member includes a bore in which one end of the coil clutch is received. The second member defines an outer periphery about which the other end of the coil clutch is received. In this orientation, the clutch will selectively slip or grip the first or second member based on the direction of rotation.
In light of the disadvantages of the prior art discussed above, it would be advantageous for a coupling device in a brake actuator mechanism to be cost effective, compact and simple to install. Additionally, the device should provide substantially constant torque resistance.